The invention relates to a flat gasket, in particular, a cylinder head gasket which has a gasket plate with at least one aperture and at least one sheet-metal layer which is provided with a bead surrounding the aperture as well as with a stopper device which prevents any excessive bead deformation and consists at least substantially of an elastomeric material which is arranged in the recess formed by the bead and fills the recess at least over part of its depth.
In the case of flat gaskets, the gasket plate of which is formed by one or several sheet-metal layers placed one on top of the other, beads surrounding apertures in a sheet-metal layer having elastic properties (customarily consisting of spring steel sheet) serve to seal against gases or liquids in that such a bead is pressed resiliently against another sheet-metal layer of the gasket plate or against components which are to be sealed in relation to one another, such as engine block and cylinder head, with the bead ridge, on the one hand, and with the two, so-called bead feet, on the other hand. Particularly in the case of cylinder head gaskets there is the risk that cracks will form in the bead during the course of operation because the width of the sealing gap between engine block and cylinder head to be sealed by the cylinder head gasket alters continuously during operation of the engine and, consequently, such a bead is constantly stressed dynamically; in order to prevent such a bead from deforming to too great an extent, i.e. from being flattened, during installation of the seal and/or during operation, so-called stoppers or stopper devices are allocated to the bead and these consist, for example, of a sheet-metal ring adjacent to the bead and welded onto the beaded sheet-metal layer, the thickness of the sheet metal of this ring being considerably less than the height of the non-deformed, i.e. non-stressed bead xe2x80x94 the stopper then prevents any excessive bead deformation and is intended to ensure that the bead is, during operation, always deformed and stressed in the elastic range.
Stopper devices of the type mentioned at the outset are, however, already known in the form of a rubber ring which is inserted into the recess formed by the bead and completely fills this recess (cf. FIG. 9 of DE-28 49 018-C); as a result of the rubber ring, the degree of bead flattening during the installation of the gasket is intended to be reduced, and polymeric material squeezed out of the bead during the installation of the gasket is intended to improve the microsealing (since rubber is an incompressible material and the bead is unavoidably flattened to some extent during the installation of the gasket, some of the material of the rubber ring will, in any case, be squeezed out of the bead during the installation of the gasket).
A similar stopper device is disclosed in EP-O 866 245-A (cf. in this case, e.g., FIGS. 3, 5, 6 and 8 to 13), wherein the stopper function is undertaken by a rigid or rather stiff ring arranged in the bead and consisting of a resin (annular rigid resin layer) which fills the recess formed by the bead over approximately half its depth.
In the case of cylinder head gaskets, it has, however, been shown that during operation of the engine bead ruptures attributable to a crack formation cannot permanently be prevented even with stopper devices of this type, provided that another, additional stopper device, e.g., in the form of the sheet-metal ring described above is not presentxe2x80x94if, however, such an additional stopper device is used, filling of the bead with elastomeric material is superfluous. The cramped space conditions often found in modern engines for cylinder head gaskets make it seem, however, to be often advisable (or even make it imperative) to do without a stopper to be arranged next to the bead.
The object underlying the invention was therefore to improve a flat gasket of the type mentioned at the outset, as known, for example, from DE-28 49 018-C, such that the endurance strength of a bead can be considerably increased in relation to dynamic stresses at right angles to the plane of the gasket plate even when dispensing with an additional, different stopper device, namely also during the occurrence not only of very slight bead deformations (flattenings).
During their efforts to solve this object the inventors have established completely surprisingly that a complete or partial filling of the bead with an elastomeric material is successful when this displays under pressure and at operating temperatures (temperatures at least corresponding, however, to the ambient temperature) flow properties (capability of plastic deformation) which are considerably above those of completely cross-linked polymeric materials. In order to delimit the present invention in practice by means of a measurement of the material properties in relation to the known state of the art, the following definition of the material properties can, in particular, be used:
The elastomeric material should be plastically deformable under pressure at a temperature corresponding at least to the ambient temperature (21xc2x0 C.) in such a manner that it has at least the following set-down behavior: With a vertical pressure load on a circular pressure-load area of a uniform, flat layer of the elastomeric material having an initial layer thickness of approximately 24 xcexcm, wherein the pressure-load area has an outer diameter of 65.4 mm and an inner diameter of 64.6 mm and the pressure load is 40 N per mm2 of the pressure-load area, the layer thickness of the elastomeric material in the pressure-load area at 21xc2x0 C. after a pressure-load duration of approximately 0.5 hours is at the most only approximately 85%, preferably only approximately 83 to approximately 37%, of the initial layer thickness.
Tests on spring steel sheets, which are provided with a bead and the recess of which formed by the bead was completely filled in one case with an almost completely cross-linked elastomeric material and in the other case with an elastomeric material with a low degree of cross-linking, have surprisingly resulted in the bead being flattened, during a dynamic pressure load, i.e. a pressure loading of the bead with a pressure altering quickly with respect to time, as is the case for a cylinder head gasket during operation of the engine, to more than double the amount without tearing when the elastomeric material with a relatively low degree of cross-linking is used (in comparison with the elastomeric material which is cross-linked almost completely).
Preferably, the bead is a so-called full-bead with a cross-section similar to a flat U.
Elastomeric materials which are preferred for the inventive gasket have, under pressure, such flow properties that the layer thickness of the elastomeric material decreases rapidly with respect to time after pressure is applied and is scarcely reduced further after approximately 0.2 hours. Such preferred, elastomeric materials display, in particular, such a set-down behavior with respect to time that after a pressure-load duration of approximately 1 hour the layer thickness is at the most only approximately 80%, in particular only approximately 80% to approximately 35% of the initial layer thickness.
The invention has a particularly advantageous effect when the elastomeric material does not completely fill the recess of the bead when the bead is not yet compressed but only over such a part of its depth that when the gasket is installed but not yet dynamically pressure-loaded the recess formed by the bead flattened somewhat in accordance with the assembly forces which are always specifically predetermined is completely filled by the elastomeric material.
The elastomeric material can be applied to the beaded sheet-metal layer only in the bead area; embodiments are, however, preferred, with which the elastomeric filling of the bead is part of an at least partial coating of the sheet-metal layer which also extends on both sides of the bead; the sheet-metal layer can, therefore, also be completely coated with the elastomeric material, namely not only on that side, on which the recess formed by the bead is located, but also on both sides.
In the case of cylinder head gaskets having a partial or full-surface coating of the beaded sheet-metal layer on one or both sides, the average layer thickness outside the bead is customarily in the order of magnitude of 18 to 28 xcexcm, in particular, from 20 to 25 xcexcm and the depth of the non-pressure-loaded bead is in the order of magnitude of 100 to 300 xcexcm, in particular, of approximately 150 xcexcm. For such cases, such a layer thickness of the elastomeric material in the recess formed by the bead is recommended that in the recess the maximum layer thickness is in the order of magnitude of 35 to 70 xcexcm, preferably from 50 to 60 xcexcm. If a coating of the sheet-metal layer with the elastomeric material also extends over areas of the sheet-metal layer located on both sides of the bead, it is generally recommended to carry out the coating such that the maximum layer thickness of the elastomeric material in the recess formed by the bead is approximately 1.5 to 3 times the average layer thickness outside the bead, above all when the average layer thickness of the elastomeric material outside the bead is approximately 18 to 28 xcexcm and preferably approximately 23 to 24 xcexcm.
During the production of sheet-metal layers provided for flat gaskets and coated partially or over their full surface with an elastomeric material, a preliminary or precursor material is customarily processed which contains an evaporatable solvent and after its hardening forms the elastomeric coating material. Known application methods for this preliminary material are sprinkling, spraying or application with the aid of the screen printing method. Since the preliminary material applied is still relatively low-viscous prior to its hardening (expelling the solvent and complete or, in the case of the present invention, partial cross-linking as a result of temperature treatment), it is suggested for a simple and inexpensive process for the production of an inventive flat gasket that a preliminary material which forms the elastomeric material after completion of the gasket and contains an evaporatable solvent be applied to the sheet-metal layer such that the preliminary material fills the recess formed by the bead to a greater degree than the elastomeric material of the finished gasket, whereupon the solvent is evaporated; in this way, it is easy to achieve the fact that the elastomeric material does not completely fill the recess formed by the bead even when, as is, for example, the case with use of the screen printing method, the preliminary material first of all fills the bead recess completely. If the preliminary material is, for example, sprinkled or sprayed on, its relatively low viscosity first of all can, however, also be utilized to achieve a greater layer thickness in the bead recess; for this purpose, it is recommended that the process be configured such that the preliminary material is applied first of all with an approximately uniform layer thickness and the solvent is not evaporated until additional preliminary material has flowed from areas on both sides of the bead into the recess formed by it.